Conveyors frequently must convey articles through environments where the ambient temperature varies over a large range. For example, bakery goods must be conveyed through ovens; canned goods are conveyed through elevated temperature sterilizers; and food products are conveyed through freezers. When conveyors operate at elevated or lowered temperatures, parts of the conveyor, for example, the belt, pulley and drive sprocket undergo thermal expansion or contraction. Design precautions must be taken to make certain that the parts can withstand the thermal expansion or contraction and that the conveyor belt remains in engagement or mesh with the drive sprocket.
A balanced spiral metal belt is a practical type of belt for use in such environments. A drive sprocket for a balanced spiral metal belt must be tailor-made to fit the unusual shaped mesh openings to effect the drive of the belt and control the tendency of side-travel. A multi-tooth sprocket which has teeth engaging every mesh opening throughout the zone of contact would be the ideal type of drive sprocket. This type of sprocket would spread the drive and guiding forces evenly across the width of the drive sprocket so that only minimal contact pressure would occur between the sprocket teeth and the tractive wire elements of the belt. One such drive sprocket is a corn cob roll wherein the sprocket is made of metal and the teeth are either machined or cast as a single piece. While this type of multitooth drive sprocket is well suited for driving balanced spiral metal belts, its cost is extremely high so that its usage is generally limited to cases where the required use justifies almost any expense. Usually a relative inexpensive belt and conveyor system is used in conjunction with a relatively expensive multi-tooth sprocket.
The use of a multi-tooth sprocket made of plastic would be less expensive than the conventional metal multi-tooth sprocket; however, problems occur in using a plastic sprocket with a metal belt because of the difference in the coefficient of thermal expansion between the two materials. The use of a continuous, integral plastic sprocket running the full width of a typical conveyor belt is precluded by the fact that even normal variations in ambient temperatures would cause the sprocket to expand laterally to the point that the teeth would not properly engage the mesh openings and tractive members of the balanced spiral belt. The lateral unit expansion of the sprocket material is accumulated and although the mesh openings of the belt might engage along one end, the belt would ride atop the teeth on the other edge.
Another technique for driving conveyor belts, which overcomes the problem of different coefficients of thermal expansion is the use compound sprockets made up of multiple single type sprockets, such as illustrated in U.S. Pat. No. Re. 30,341 wherein one sprocket is fixed against axial movement while remaining sprockets are free to move axially. The expense of constructing the single sprockets limits the number of the multiple single sprockets which can be used in a system, thereby limiting the amount of engagement the sprocket makes with the belt.
The present invention alleviates these problems of the prior art drive sprockets so that a relatively inexpensive drive sprocket is constructed with multiple teeth to engage a substantial portion of the width of the belt; and the drive sprocket does not have the problem of belt disengagement even though the sprocket is made of a material different from that of the belt and is used under varying ambient temperature conditions.